1. Field of the Invention
The present invention relates to a transparent or translucent glass-ceramic plate (these terms include plates known as semitransparent plates having, for example, coloration in the bulk) intended, especially, to cover or receive heating elements, in particular intended to serve as a hob, the heating elements associated with this hob being, for example, heat sources for induction heating, radiant heat sources, halogen heat sources, etc.
2. Description of the Background
Sales of glass-ceramic hobs have been continuing to grow over the last few years. This success is explained in particular by the attractive appearance of such hobs and by the ease of cleaning them.
It will be recalled that a glass-ceramic is originally a glass, called a precursor glass, the specific chemical composition of which allows controlled crystallization to be induced by suitable heat treatments, called ceramification. This partly crystallized specific structure gives the glass-ceramic unique properties.
At the present time, there are various types of glass-ceramic plate, each variant being the result of extensive research and many tests, given that it is very difficult to make modifications to these plates and/or to the process for obtaining them without risking an unfavorable effect on the desired properties. To be able to be used as a hob, a glass-ceramic plate must especially have a transmission in the wavelength in the visible range that is both low enough to mask at least some of the underlying heating elements when not in use and high enough so that, for the sake of safety, the user can visually detect the heating elements that are in operation without being dazzled, and the operating signs or displays giving the power of the heat sources. Generally, it must also have a high transmission in the wavelength in the infrared range, in particular for use with radiant or halogen heat sources.
The first plates developed were thus plates of dark color. More recently, other plates have been developed of lighter color (in particular white), and having, for example, a haze of at least 50% (as described in Patent FR 2 766 816), the consideration of the aforementioned criteria (such as the detection of the heating elements in operation without dazzling or unappealing appearance of said elements) sometimes being carried out to the detriment of other aspects (such as the clear viewing of optional displays placed under the plate).